Integrated circuit, IC, applications often require a reference current. Typically, such reference currents are provided by integrated reference currents circuits. With the continued increase in device densities within integrated circuits, the required precision and stability of such reference currents also continues to increase. Furthermore, such reference currents are required to be temperature independent.
FIG. 1 illustrates a circuit diagram of an example of a conventional integrated current generator circuit 100 for generating a reference current IOUT 105. IOUT 105=ITI 110 by virtue of a current mirror arrangement comprising MOS (Metal Oxide Semiconductor) devices M1 120, M2 122 and M3 124. BJT (Bipolar Junction transistor) devices Q1 130 and Q2 132 are configured in an asymmetrical current mirror arrangement, with resistance R2 142 providing a voltage difference between their respective base terminals. The base-emitter voltage of Q1 130 (VbeQ1) is applied across resistance R1 140, thus the current through R1 140 is equal to VbeQ1/R1. Assuming that the base current of Q1 130 is negligible (i.e. much less than ITI 110), the current through R2 142 is equal to the current through R1 140 due to feedback provided by MOS device M4 126. So, the voltage applied to the base of Q2 132 is (R2+R1)*VbeQ1/R1. The voltage at the emitter of Q2 132 (VeQ2) is lower than at the base of Q2 132 by VbeQ2. Accordingly, VeQ2=((R2+R1)*VbeQ1/R1)−VbeQ2. Devices Q1 130, Q2 132, M1 120 and M2 122 are sized in such a way as to provide a Q2 emitter current density that is N times lower than a Q1 emitter current density. Accordingly:VbeQ1−VbeQ2=VT*ln(N)  [Equation 1]where VT is a thermal potential k*T/q, and “k” is a Boltzmann's constant, “q” is the charge of an electron, and “T” is an absolute temperature, in degrees of Kelvin.
The voltage at the emitter of Q2 132 may be written as following:
                              V                      eQ            ⁢                                                  ⁢            2                          =                                                                                                  R                    ⁢                                                                                  ⁢                    1                                    +                                      R                    ⁢                                                                                  ⁢                    2                                                                    R                  ⁢                                                                          ⁢                  1                                            ·                              V                                  beQ                  ⁢                                                                          ⁢                  1                                                      -                          V                              beQ                ⁢                                                                  ⁢                2                                              =                                                                      (                                      1                    +                                                                  R                        ⁢                                                                                                  ⁢                        2                                                                    R                        ⁢                                                                                                  ⁢                        1                                                                              )                                ·                                  V                                      beQ                    ⁢                                                                                  ⁢                    1                                                              -                              V                                  beQ                  ⁢                                                                          ⁢                  2                                                      =                                                                                R                    ⁢                                                                                  ⁢                    2                                                        R                    ⁢                                                                                  ⁢                    1                                                  ⁢                                  V                                      beQ                    ⁢                                                                                  ⁢                    1                                                              +                              (                                                      V                                          beQ                      ⁢                                                                                          ⁢                      1                                                        -                                      V                                          beQ                      ⁢                                                                                          ⁢                      2                                                                      )                                                                        [                  Equation          ⁢                                          ⁢          2                ]            
Substituting Equation 1 into Equation 2 gives:
                              V                      eQ            ⁢                                                  ⁢            2                          =                                                            R                ⁢                                                                  ⁢                2                                            R                ⁢                                                                  ⁢                1                                      ⁢                          V                              beQ                ⁢                                                                  ⁢                1                                              +                                                    k                ·                T                            q                        ·                          ln              ⁡                              (                N                )                                                                        [                  Equation          ⁢                                          ⁢          3                ]            
According to Equation 3, the voltage at the emitter of Q2 is a sum of two terms. The first term is proportional to VbeQ1 voltage, having a negative temperature coefficient. The second term is proportional to absolute temperature T. When these two terms are taken in the right proportion, determined by the R2/R1 ratio, their sum may be almost independent of temperature.
The voltage at the emitter of Q2 132 is applied to the (R3+Rabs<1>+Rabs<0>) calibration resistance 144. The temperature independent current ITI 110 is equal to:
                              I          TI                =                                            V                              eQ                ⁢                                                                  ⁢                2                                                                    R                ⁢                                                                  ⁢                3                            +                              Rabs                ⁢                                  〈                  1                  〉                                            +                              Rabs                ⁢                                  〈                  0                  〉                                                              =                                    1                                                R                  ⁢                                                                          ⁢                  3                                +                                  Rabs                  ⁢                                      〈                    1                    〉                                                  +                                  Rabs                  ⁢                                      〈                    0                    〉                                                                        ·                          (                                                                                          R                      ⁢                                                                                          ⁢                      2                                                              R                      ⁢                                                                                          ⁢                      1                                                        ·                                      V                                          eQ                      ⁢                                                                                          ⁢                      1                                                                      +                                                                            k                      ·                      T                                        q                                    ·                                      ln                    ⁡                                          (                      N                      )                                                                                  )                                                          [                  Equation          ⁢                                          ⁢          4                ]            
By adjusting the ratio between R2 142 and R1 140, the temperature coefficient of ITI 110 may be adjusted. By adjusting the resistance of the calibration circuit 144, the absolute value of ITI 110 may be adjusted, or ‘trimmed’ to achieve a desired reference current IOUT 105.
FIG. 2 illustrates an example of IOUT 105 versus temperature dependence for different states of calibration achieved through conventional adjusting of the ratio between R2 142 and R1 140 for the conventional integrated current generator circuit 100 of FIG. 1.
As apparent from Equation 4 above, when the temperature coefficient for such a conventional integrated current generator circuit 100 is trimmed (by adjusting the ratio between R2 142 and R1 140), the absolute value of ITI 110, and thus of IOUT 105, is changed as well. To trim both the temperature coefficient and the absolute value, the following test procedure should be implemented:                i) Measure IOUT value at a first temperature (T1) from a range; store the measured value IOUT (T1) either in external memory (tester), or in internal memory (non-volatile die memory, fuses, etc.).        ii) Measure IOUT value at a second temperature (T2) from a range; calculate the temperature coefficient, implement TC calibration based on temperature coefficient calculated.        iii) Trim the absolute value of IOUT; assuming the TC is minimized, the absolute value calibration may be implemented at the same temperature T2.        
A problem with such a calibration procedure is the need to store the value IOUT (T1). If this value is stored in external memory (e.g. within test equipment), all of the IC devices in a lot have to be serialized (numbered and tracked). If this value is stored in internal memory (i.e. on-die), it requires additional die size.
Furthermore, using a look-ahead procedure (a simple search through all trim bit combinations to find the best one) for temperature coefficient calibration is prohibitively complicated for such a calibration procedure. When a look-ahead procedure is implemented at a given, single temperature, it is quite simple and straightforward. The simple search through all trim bit combinations to find and apply a specific combination that achieves the target IOUT may be easily and efficiently implemented. However, when more than one (i.e. two in the above procedure) test insertion at multiple (i.e. two in the above procedure) different temperatures are required, the look-ahead procedure becomes prohibitively complicated because one needs to store not just a single number (e.g. the result of the IOUT measurement at T1), but all data measured (i.e. two arrays of numbers corresponding to both T1 and T2 in the above procedure), and then to search through all trim bit combinations to find and apply a specific combination that achieves the target IOUT taking into account all measured data. Performing such a look-ahead procedure for all IC devices in a lot during mass production is prohibitively complicated.
Accordingly, blind calibration using typical step value from a calibration table is typically used instead. For example, assuming IOUT is measured at T1 and the result is stored. IOUT is then re-measured at T2. The Temperature coefficient may then be calculated as [IOUT(T1)−IOUT(T2)]/[T1−T2]. After that, calibration may be performed using some assumption about best trim bit combination. However, being resistive-dependent, the trim step value is not absolutely precise; it depends on process variation as well. Accordingly, trim errors are possible when using blind calibration. The result of blind calibration may be validated only after the calibration is performed, with the calibrated circuit being re-measured again at T1 & T2. However, such validation is not practical, because it is too expensive to perform multiple thermal cycling during mass production. As such, the result of blind calibration may not be as accurate and consistent with process variation as a look-ahead procedure.